1. Field of the Invention
The present invention relates to an apparatus or structure for promoting intermixing of heated coolant streams within a upper plenum of a nuclear reactor, which plenum is disposed above a reactor core in which heat exchange takes place between fuel rods and a coolant such as light water and which has high and low temperature regions where the coolant is heated to high and low temperatures, respectively. More particularly, the invention is concerned with a structural configuration and disposition of internal component members disposed within the upper plenum at locations substantially corresponding to an outer periphery of a fuel region.
2. Description of Related Art
In a power reactor, heat generated internally of fuel rods is transferred to a coolant such as light water or the like for utilization as energy. In other words, heat exchange is carried out between the nuclear fuel and the coolant. For a better understanding of the invention, background techniques thereof will be reviewed below. FIG. 3 of the accompanying drawings shows an internal structure of a pressurized water reactor, a typical nuclear reactor. Referring to the figure, there are accommodated within a nuclear reactor vessel 10 core internals inclusive of nuclear fuel assemblies, control rods, control rod cluster guide tubes, a coolant and others. In general, the nuclear reactor vessel 10 is integrally provided with inlet nozzles 11 and outlet nozzles 12 for the reactor coolant which is light water, wherein a core barrel 30 is suspended vertically within the nuclear reactor vessel 10. The number of the inlet nozzles 11 and the outlet nozzles 12, respectively, coincide with the number of coolant circulation loops which in turn is determined in dependence on the output power rating of the reactor. Ordinarily, the number of the inlet nozzles 11 as well as that of the outlet nozzles 12 is in a range of two to four. The inlet nozzles 11 as well as the outlet nozzles 12 are installed in the nuclear reactor vessel in a circumferential direction with predetermined distance therebetween. Disposed within the core barrel 30 at a lower portion thereof are a lower core support plate 32 and a lower core plate 31, each extending in a horizontal direction. A bottom plenum 41 is defined beneath the lower core support plate 32.
Mounted on the lower core plate 31 are a large number of fuel assemblies 33 which are disposed adjacent to one another to thereby constitute a reactor core. Further disposed above the fuel assembly 33 is an upper core plate 21 which is supported by an upper core support plate 20 by way of upper core support columns 23. The fuel assemblies 33 are pressed downwards by means, of the upper core plate 21 so that the fuel assemblies 33 are prevented from displacing upwards under the influence of buoyancy exerted by the flowing coolant. A plurality of control rod cluster guide tubes 22 are supported at lower ends thereof on the upper surface of the upper core plate 21 by means of supporting pins or the like (not shown). The control rod cluster guide tubes 22 extend upwardly through and beyond the upper core support plate 20. By withdrawing the control rod clusters (not shown either) from the reactor core through the medium of the control rod cluster guide tubes 22 or inserting the control rod clusters into the fuel assemblies 33 of the reactor core through the control rod cluster guide tubes 22, the thermal output of the reactor core can be adjusted.
FIG. 4 is a fragmentary enlarged view showing a structure above the fuel assembly 33. As can be seen in this figure, the upper core plate 21 and the upper core support plate 20 are interconnected by the upper core support columns 23 in order to ensure structurally high strength or robustness. Furthermore, the control rod cluster guide tubes 22 extending through the upper core support plate 20 are fixedly secured to the upper core support plate 20. Thus, the control rod cluster guide tubes 22 are protected against displacement or dislocation in a lateral or transverse direction. Defined between the upper core plate 21 and the upper core support plate 20 interconnected as mentioned above is a upper plenum 40.
Next, description will be directed to the flows or streams of light water employed as the coolant within the nuclear reactor vessel 10 realized in the structure described above. Referring to FIGS. 3 and 4, light water of low temperature fed to the nuclear reactor vessel 10 by way of the inlet nozzles 11 follows flow paths of such patterns as indicated by arrows in these figures. More specifically, light water fed to the nuclear reactor vessel 10 through the inlet nozzle 11 flows at first downwardly through an annular space defined between the outer surface of the core barrel 30 and the inner wall of the nuclear reactor vessel 10. The flowing direction of the light water is forced to turn upwards within the bottom plenum 41. Thereafter, light water flows into the reactor core after passing through the lower core support plate 32 and the lower core plate 31. Within the reactor core, light water flows upwardly substantially in parallel. In the course of flowing through the reactor core, heat generated by the fuel rods of the fuel assemblies is deprived of by light water, which results in temperature rise thereof. After passing through the upper core plate 21, the flowing direction of light water changes to a horizontal or transverse direction, being deflected under the stop action of the upper core support plate 20. Finally, light water leaves the nuclear reactor vessel 10 through the outlet nozzle 12 to be supplied to a steam generator (not shown) by way of an outlet pipe 42.
At this juncture, it should be mentioned that the reactor core which is constituted by a plurality of fuel assemblies 33 ordinarily undergoes periodical maintenance for fuel exchange such that about one third of the fuel is exchanged at the end of every operation cycle. Consequently, the core is constituted by three groups of fuel assemblies in correspondence to three cycles which differ from one another in respect to the degree of burn-up (hereinafter referred to as the burn-up degree). Thus, the output powers of the fuel assemblies 33 differ from one to another assembly in dependence on the burn-up degrees. Besides, in the core region where the control rods have been loaded for controlling the output power of the reactor as well as in the outer peripheral region of the core where leakage of neutron fluxes externally of the reactor occurs, there prevails neutron flux distribution of low density when compared with that in a center region of the core. As a consequence, power outputted from the outer peripheral portion or region of the reactor core is low when compared with that of the center region of the core.
Such being the circumstances, the flow behavior of light water within the reactor core will now be analyzed in more detail. Under the influence of the different neutron flux distributions within the reactor core such as described above, a stream (indicated by an arrow d in FIG. 4) of light water flowing through the center region of the core where the nuclear fission is vigorous is heated up to a relatively high temperature, whereon the light water heated to a high temperature leaves the core to flow into the upper plenum 40. In succession, high temperature light water flows along and through the control rod cluster guide tubes 22 disposed within the upper plenum 40 to impinge on the lower surface of the upper core support plate 20, whereby the flowing direction of the light water is deflected so that it flows substantially transversely through the upper plenum 40. Finally, light water flows out from the reactor through the outlet nozzle 12, as indicated by arrows e, f and g in FIG. 4.
On the other hand, a stream (indicated by an arrow a in FIG. 4) of light water flowing through the outer peripheral region of the core where the neutron flux density is relatively low will is heated to a relatively low temperature when compared with the stream d of the light water flowing through the center region of the core. Of course, light water of the stream a flows into the upper plenum 40. However, in this conjunction, it is noted that the peripheral stream of light water flowing toward the outlet nozzle 12 within the upper plenum 40 follows such flow paths as indicated by arrows b and c in FIG. 4. As can be seen in this figure, the peripheral stream light water flows through a region within the upper plenum 40 where flow resistance is relatively low because essentially no obstacles are present in this region located at the inner side of the core barrel 30. Consequently, low temperature light water can flow into the outlet nozzle 12 straightforwardly.
For the reasons mentioned above, within the outlet pipe 42 connected to the outlet nozzle 12, the peripheral light water stream of relatively low temperature tends to flow along a bottom portion of the outlet pipe 42, as indicated by the arrow c, while the center stream (i.e., light water stream having passed through the center region of the core) of a relatively high temperature follows a flow path along a top or ceiling portion of the outlet pipe 42, as indicated by the arrow g. In another words, there are formed within the outlet pipe 42 and the outlet nozzle 12 an upper layer or stream of high temperature light water and a lower layer or stream of low temperature light water. In other words, the flow of light water is stratified into layers of different temperatures within the outlet pipe 42 and the outlet nozzle 12. As a result of this, the light water flow assumes such a temperature distribution that a remarkable temperature gradient makes appearance in the coolant flow within the outlet pipe as well as succeeding pipes. Besides, when the light water streams having the temperature difference as mentioned above pass through reactor internals, vortexes are produced, involving fluctuation in the temperature of light water. These phenomena are likely to provide obstacle for measurement of a mean temperature (temperature on an average) of the coolant with accuracy. Parenthetically, such measurement is usually carried out by measuring the temperature of light water flowing through the piping system.
As the measures for coping with the undesirable phenomena described above, there has already been proposed a structure for promoting intermixing of heated coolant streams by changing the flow directions of the peripheral stream by means of internal component members each of which is realized in a specific structural configuration with a view to reducing the temperature difference between the heated coolant streams, as is disclosed in Japanese Patent Application Laid-open No. 9-72985. FIG. 5 of the accompanying drawings shows a hitherto known or conventional structure which has been developed to this end. As can be seen in the figure, slot-formed tubes 24 are disposed substantially along and over the whole outer periphery of the heat generating region. By virtue of the arrangement mentioned above, a part of light water of low temperature flowing out from the outer peripheral region of the core, as indicated by an arrow a in FIG. 5, can flow into the slot-formed tubes 24 mounted by appropriate means, as indicated by an arrow b' in FIG. 5, to enter the upper plenum 40 through the slots formed in the upper region of the slot-formed tubes 24 to be thereby deflected toward the outlet nozzle 12, as indicated by arrow c'. Owing to such flow pattern of light water or coolant streams, a part of the peripheral light water stream of a relatively low temperature flowing through the slot-formed tube 24 and the center light water stream of a high temperature passed through the center core region (indicated by arrows d and e) can intermingle or intermix with each other to an appropriate extent within the upper plenum 40. As a result of this, the temperature distribution of the coolant, flowing into the outlet nozzle 12 can be made uniform. Thus, light water can flow through the outlet pipe 42 with lesser temperature gradient.
However, the structure in which the low temperature light water is introduced into the slot-formed tubes 24 each having an increased outer diameter in order to guide the coolant upwardly to a height level near to the top of the outlet nozzle 12 from the height level of the upper core plate 21 necessarily becomes bulky because of the large outer diameter as compared with that of the upper core support columns 23 which are of course incapable of allowing the coolant to flow therethrough. As a result of this, channels for transverse or cross flows of light water toward the outlet nozzle 12 within the upper plenum 40 become narrow, involving increased flow resistance to the flow of the coolant. As a result of this, hydrodynamic load (fluid load) applied to the upper core support columns 23 and the control rod cluster guide tubes 22 increases, which in turn may exert adverse influence to the structural integrity of the control rod cluster guide tubes 22 among others.
Furthermore, in case the slot-formed tubes 24 are disposed between the upper core plate 21 and the upper core support plate 20 at all the available positions corresponding to the outer periphery of the reactor core except for the locations of the control rod cluster guide tubes disposed above the fuel assembly, the channel blockage ratio increases significantly as well. As a result of this, the flow resistance presented by the internal component members installed within the upper plenum 40 increases, whereby the hydrodynamic load applied to the control rod cluster guide tubes 22 will become about twice as high as the hydrodynamic or fluid load in the structure in which no slot-formed tubes 24 are provided. Such increase of the hydrodynamic load is undesirable from the viewpoint of safety as well in view of the fact that the control rod cluster guide tubes 22 are mounted on the upper core plate 21 at the lower ends thereof simply by means of supporting pins or the like, as described previously.